The present invention relates generally to semiconductor circuits, and in particular to a method for the fabrication of microtransformers which can be used in a system-on-chip power supply.
The system-on-chip concept refers to a system in which, ideally, all the necessary integrated circuits are fabricated on a single die or substrate. Various packaging schemes have been proposed to achieve integration of chips with different functionalities in a single package by mounting them on a silicon interposer to form a circuit module. For example, the simplest scheme is the chip-on-chip module, in which a microprocessor chip and a memory chip are stacked together, face to face or through a silicon interposer, using micro bump bonding (MBB) technology.
Integrating all system components into one chip or a plurality of chips in a single module affords a smaller product size, higher speed, and increased reliability. Power consumption remains, however, a critical issue, especially for portable devices having complex circuitry, which requires an always increasing number of devices to be integrated on one chip.
In an effort to reduce the power consumption, power supply components, such as DC-DC converters, intelligent power LSIs, and thin film magnetic devices, have been integrated into one chip. Since a typical DC-DC converter consists of semiconductor devices, resistors, capacitors, and electromagnetic components, such as transformers and inductors, among others, a key issue for the system-on-chip DC-DC converter is integrating both semiconductor and electromagnetic devices into a chip, while reducing the size of the electromagnetic components, which tend to occupy a large amount of space.
For example, U.S. Pat. No. 5,279,988 to Saadat and Thomas teaches a processes for fabrication of microcomponents integrated circuits, including microtransformers and microinductors, using multilevel metallization involving six layers of insulators and a coil winding.
U.S. Pat. No. 5,583,474 to Mizoguchi et. al. discloses xe2x80x9ca planar magnetic elementxe2x80x9d consisting of a pair of planar spiral coils sandwiched by two thin magnetic films to form an inductor or a transformer.
Similarly, U.S. Pat. No. 5,519,582 to Matsuzaki discloses a magnetic induction coil directly mounted on, and integrated with, a semiconductor wafer containing integrated circuitry. Grooves are etched in the reverse face of the wafer substrate, an insulating film is applied, and conducting materials fill the grooves forming therefore the coil.
The implementation of a truly high performance system-on-chip DC-DC converter with electromagnetic elements poses various problems, mainly because of the growing demand for increased efficiency at high frequency operations. High frequency operations are highly desirable for electromagnetic elements since they permit a decrease in the size of the device while affording the same reactance. Yet, at frequencies higher than 1 MHz, operating frequency increases tend to have a detrimental effect on the efficiency of the devices. Multilayered integrated circuit structures for forming electromagnetic components have attempted to address the efficiency issue, but have reached only limited results.
Another disadvantage of electromagnetic devices operating at high frequencies is the limitation posed by the width of the winding conductor. Because the electromagnetic element coil is formed by a thin film conductor, its width must be limited to form the desired fine pitch structure. Consequently, the current capacity of the magnetic induction is also limited and, in turn, limits the current density in the coil.
Further, planar electromagnetic elements fabricated today are not yet small enough to be integrated with other circuit elements, making it practically impossible to manufacture sufficiently small system-on-chip power supplies.
There is needed, therefore, a method for further downsizing of electromagnetic elements, for example coils and microtransformers on ICs operating at high frequencies with high efficiency, low losses, and high magnetic permeability. An electromagnetic element for use in a circuit section that will only slightly influence other components of the circuit, will have a sufficiently high current capacity and high inductance, and will occupy a minimal substrate area is also needed, as well as a simple process for fabricating such an electromagnetic element.
The present invention provides an integrated circuit microtransformer capable of operating at high frequencies with high efficiency, low losses, and high magnetic permeability.
The microtransformer of the present invention uses a silicon substrate with a pair of through-holes on which an insulating silicon oxide layer is first deposited on all surfaces of the substrate. A magnetic film, such as Permalloy or others, is further deposited on the silicon oxide layer followed by the application of another insulating layer. Coils are fabricated next by patterned deposition on both sides of the substrate and through the holes. The pair of through-holes allows the winding of a single coil or the winding of primary and secondary coils to pass through the holes and thus to reside on both surfaces of the substrate. The typical through-hole size is approximately 1 mm, which can accommodate, for example, up to 83 windings of 8xcexcm lines on a 12xcexcm spacing. The coils can be, for example, single coils, or primary or secondary coils of a transformer structure, with secondary coils having one or more output taps to supply different output voltages. For better flux closure, various magnetic layers and insulators can be deposited on top of the windings. The primary and secondary windings of a transformer can also be wound through the holes, but at different levels.